PET is a molecular imaging modality that utilizes trace amounts of radiolabeled molecules (probes) to target, measure, and image biological processes in tissue concentrations down to the range of nano-&#8208; to picomoles. Imaging technologies from SOFIE and other companies allow the same probes to be used in cell cultures, mice, and patients to integrate findings from each of these settings with other basic science and clinical results. UCLA investigators developed, and licensed to SOFIE for further commercialization, a family of nucleoside analog probes ([18F]FAC) that are transported and trapped intracellularly by dCK phosphorylation. SOFIE and others can now reference UCLA INDs for these probes, allowing further investigation into [18F]FAC as diagnostic agents for assaying dCK enzyme activity in predicting tumor responses to dCK dependent prodrugs. One potential roadblock to widespread use of this probe is that no commercial radiosynthesizer exists for synthesizing [18F]FAC for pre-&#8208;clinical and clinical investigations. This proposal will also develop a commercial, cGMP-&#8208;friendly, automated radiosynthesizer to produce [18F]FAC. Therefore, the deliverables of this project are approaches and technologies broadly applicable to predicting tumor responses to dCK dependent nucleoside prodrugs that are widely used in treatment of cancer and under investigation in a large number of trials.